DHC Working Group Ted Lemon
INTERNET DRAFT Nominum
Expires: July 2004 Bill Sommerfeld
Internet Draft Sun Microsystems
Document: <draft-ietf-dhc-3315id-for-v4-01.txt>
Category: Standards Track January, 2004
Node-Specific Client Identifiers for DHCPv4
Status of this Memo
This document is a submission by the Dynamic Host Configuration
Working Group of the Internet Engineering Task Force (IETF). Comments
should be submitted to the dhcwg@ietf.org mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
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Abstract
This document specifies the format that is to be used for encoding
DHCPv4 [RFC2131 and RFC2132] client identifiers, so that those
identifiers will be interchangeable with identifiers used in the
DHCPv6 protocol [RFC3315].
Introduction
This document specifies the way in which DHCPv4 clients should
identify themselves. DHCPv4 client implementations that conform to
this specification use a DHCPv6-style DHCP Unique Identifier (DUID)
encapsulated in a DHCPv4 client identifier option. This supersedes
the behaviour specified in RFC2131 and RFC2132.
The reason for making this change is that as we make the transition
from IPv4 to IPv6, there will be network devices that must use both
DHCPv4 and DHCPv6. Users of these devices will have a smoother
network experience if the devices identify themselves consistently,
regardless of the version of DHCP they are using at any given
moment. Most obviously, DNS updates made by the DHCP server on
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behalf of the client will not be handled correctly. This change
also addresses certain limitations in the functioning of
RFC2131/2132-style DHCP client identifiers.
This document first describes the problem to be solved. It then
states the new technique that is to be used to solve the problem.
Finally, it describes the specific changes that one would have to
make to RFC2131 and RFC2132 in order for those documents not to
contradict what is described in this document.
1.0 Applicability
This document updates RFC2131 and RFC2132. DHCPv4 servers
implementations SHOULD conform to this document. DHCPv4 clients on
network devices that are expected to support DHCPv6 in the future
SHOULD conform to this document. This document makes no changes to
the behavior of DHCPv6 clients or servers.
DHCPv4 clients and servers that are implemented according to this
document should be implemented as if the changes specified in
section 4.3 and 4.4 have been made to RFC2131 and RFC2132.
2.0 Problem Statement2.1. Client identities are ephemeralRFC2132 recommends that client identifiers be generated by using
the permanent link-layer address of the network interface that the
client is trying to configure. In cases where a network interface
is removed from the client computer and replaced with a different
network interface with a different permanent link-layer address,
the identity of the client changes. The client loses its IP
address and any other resources associated with its old identifier
- for example, its domain name as published through the DHCP
server.
2.2. Clients can accidentally present multiple identities
Consider a DHCP client that has two network interfaces, one of
which is wired and one of which is wireless. There are three
interesting cases here:
(a) Each network interface is attached to a different link.
(b) Both network interface are connected to the same link.
(c) Only one network interface is attached to a link.
Case (a) is problematic, and is beyond the scope of this document.
Even the full implications of cases (b) and (c) are beyond the
scope of this document. However, it seems safe to point out that
cases (b) and (c) are very common in practice, because many
devices such as laptop computers that are popular at the time of
this writing have both wireless and wired network interfaces that
are installed simultaneously. Both wired and wireless have
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advantages - wired has the advantage of speed, and wireless the
advantage of mobility.
So it seems likely that there will be devices that are in states
(b) and (c) frequently, and indeed frequently make transitions
between these states. If the DHCP client that configures these
devices uses the link-layer address of each device as an
identifier, the two devices will appear to the DHCP server to be
different nodes, and thus will be assigned different IP addresses,
and, in state (b), only one of the two devices will be reachable
through the domain name registered by the DHCP server.
Furthermore, if a device in state (b) makes the transition to
state (c), it is quite possible that the lease for the device that
has lost connectivity will remain valid for some time, and will be
the one that got the registered domain name. In this case, the
client will not be reachable through its registered domain name.
2.3. RFC2131/2132 and RFC3315 identifiers are incompatible
The 'client identifier' option is used by DHCP clients and servers
to identify clients. In some cases, the value of the 'client
identifier' option is used to mediate access to resources (for
example, the client's domain name, as published through the DHCP
server). RFC2132 and RFC3315 specify different methods for
deriving client identifiers. These methods guarantee that the
DHCPv4 and DHCPv6 identifier will be different. This means that
mediation of access to resources using these identifiers will not
work correctly in cases where a node may be configured using DHCPv4
in some cases and DHCPv6 in other cases.
2.4. RFC2131 does not require the use of a client identifierRFC2131 allows the DHCP server to identify clients either by
using the client identifier option sent by the client, or, if the
client did not send one, the client's link-layer address. Like
the client identifier format recommended by RFC2131, this suffers
from the problems previously described in (2) and (3).
3. Solutions
The solution to problem (2.1) is to use a DHCP client identifier
that is persistent - not tied to a particular piece of removable
network hardware. Then, when network hardware is swapped in and
out, the client identifier does not change, and thus the client has
a consistent IP address and consistent use of whatever resources
have been associated with its identifier.
It is worth noting that in case (2.1), it is harmless for the
device to use the same client identifier on both interfaces - in
this case, the DHCP server or servers serving these two links will
see the two network interfaces as distinct because they are
connected to different links, even though they use the same
identifier.
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The solution to problem (2.2) is the same. Although it is beyond
the scope of this document to say how a DHCP client supporting two
network interfaces in this way would provide a smooth user
experience, it does seem safe to say that it will need to use a
persistent DHCP client identifier that is the same across the
interfaces that it configures.
In case (2.2), if both interfaces are connected to the same link,
the DHCP server will see requests sent by the client on each
interface as being from the same client, and will only allocate one
lease to that client. A DHCP client that sends the same client
identifier on two interfaces will need to be prepared for the
possibility that both interfaces will be assigned the same IP
address. It could do this either by shutting down one interface in
this case, or it could use some more complicated strategy. It is
beyond the scope of this document to describe the details of how
this should be done. Obviously, to get the benefit of this
strategy, transitions from one device to the other must go
unnoticed by the user.
The solution to problem (2.3) is to use the DHCP Unique Identifier
as defined in RFC3315 as a client identifier. The DUID provides
several different ways of producing persistent DHCP client
identifiers, at least one of which is likely to be appropriate for
any particular sort of network device. So it turns out that this
also solves problems (1) and (2).
The solution to problem (2.4) is to deprecate the use of the
contents of the chaddr field in the DHCP packet as a means of
identifying the client.
4. Implementation Requirements
Here we specify changes to the behavior of DHCP clients and
servers. We also specify changes to the wording in RFC2131 and
RFC2132. DHCP clients, servers and relay agents that conform to
this specification must implement RFC2131 and RFC2132 with the
wording changes specified in sections 4.3 and 4.4.
4.1. DHCP Client behavior
DHCP clients conforming to this specification MUST use stable DHCP
node identifiers in the dhcp-client-identifier option. DHCP
clients MUST NOT use client identifiers based solely on layer two
addresses that are hard-wired to the layer two device (e.g., the
ethernet MAC address) as suggested in RFC2131, except as allowed in
section 9.2 of RFC3315. DHCP clients MUST send a 'client
identifier' option containing a DHCP Unique Identifier, as defined
in section 9 of RFC3315.
The general format of the DHCPv4 'client identifier' option is
defined in section 9.14 of RFC2132. To send a
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To send a DUID in a DHCPv4 'client identifier' option, the type of
the 'client identifier' option should be 255. The type field is
immediately followed by the DUID. The format of the 'client
identifier' option is as follows:
Code Len Type DHCP Unique Identifier
+-----+-----+-----+-----+-----+-----+-----+---
| 61 | n | 255 | d1 | d2 | d3 | d4 | ...
+-----+-----+-----+-----+-----+-----+-----+---
Any DHCPv4 or DHCPv6 client that conforms to this specification
SHOULD provide a means by which an operator can learn what DUID the
client has chosen. Such clients SHOULD also provide a means by
which the operator can configure the DUID. A device that is
normally configured with both a DHCPv4 and DHCPv6 client SHOULD
automatically use the same DUID for DHCPv4 and DHCPv6 without any
operator intervention.
DHCP clients that support more than one network interface SHOULD
use the same client identifier on each interface. Such DHCP
clients SHOULD be prepared for the possibility that the DHCP server
will allocate the same IP address to both interfaces.
4.2. DHCPv4 Server behavior
This document does not require any change to DHCPv4 or DHCPv6
servers that follow RFC2131 and RFC2132. However, some DHCPv4
servers can be configured not to conform to RFC2131 and RFC2131, in
the sense that they ignore the 'client identifier' option and use
the client's hardware address instead. Some DHCP servers do not
take into account the possibility that the same client identifier
may be used on separate links, and thus will behave incorrectly
when a DHCP client acquires leases on two different IP addresses on
two different links at the same time.
DHCP servers that conform to this specification MUST use the
'client identifier' option to identify the client if the client
sends it. DHCP servers MUST assume that when a lease on an IP
address is bound to a particular DHCP client identifier, all other
still-valid leases on IP addresses bound to that client identifier
are still in use.
DHCP servers MAY use administrator-supplied values for chaddr and
htype to identify the client in the case where the administrator is
assigning a fixed IP address to the client, even if the client
sends an client identifier option. This is ONLY permitted in the
case where the DHCP server administrator has provided the values
for chaddr and htype, because in this case if it causes a problem,
the administrator can correct the problem by removing the offending
configuration information.
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Internet-Draft Node-specific Identifiers for DHCPv4 January 20034.3. Changes from RFC2131
In section 2 of RFC2131, on page 9, the text that reads "; for
example, the 'client identifier' may contain a hardware address,
identical to the contents of the 'chaddr' field, or it may contain
another type of identifier, such as a DNS name" is deleted.
In section 4.2 of RFC2131, the text "The client MAY choose to
explicitly provide the identifier through the 'client identifier'
option. If the client supplies a 'client identifier', the client
MUST use the same 'client identifier' in all subsequent messages,
and the server MUST use that identifier to identify the client. If
the client does not provide a 'client identifier' option, the
server MUST use the contents of the 'chaddr' field to identify the
client." is replaced by the text "The client MUST explicitly
provide a client identifier through the 'client identifier'
option."
In the same section, the text "Use of 'chaddr' as the client's
unique identifier may cause unexpected results, as that identifier
may be associated with a hardware interface that could be moved to
a new client. Some sites may choose to use a manufacturer's serial
number as the 'client identifier', to avoid unexpected changes in a
clients network address due to transfer of hardware interfaces
among computers. Sites may also choose to use a DNS name as the
'client identifier', causing address leases to be associated with
the DNS name rather than a specific hardware box." is replaced by
the text "The DHCP client MUST NOT rely on the 'chaddr' field to
identify it."
In section 4.4.1 of RFC2131, the text "The client MAY include a
different unique identifier" is replaced with "The client MUST
include a unique identifier".
In sections 3.1, item 4 and 6, 3.2 item 3 and 4, and 4.3.1, where
RFC2131 says that 'chaddr' may be used instead of the 'client
identifier' option, the text that says "or 'chaddr'" and "'chaddr'
or" is deleted.
Note that these changes do not relieve the DHCP server of the
obligation to use 'chaddr' as an identifier if the client does not
send a 'client identifier' option. Rather, they oblige clients
that conform with this document to send a 'client identifier'
option, and not rely on 'chaddr' for identification. DHCP servers
MUST use 'chaddr' as an identifier in cases where 'client
identifier' is not sent, in order to support old clients that do
not conform with this document.
4.4. Changes from RFC2132
The text in section 9.14, beginning with "The client identifier MAY
consist of" through "that meet this requirement for uniqueness." is
replaced with "the client identifier consists of a type field whose
value is normally 255, followed by a two-byte type field, followed
by the contents of the identifier. The two-byte type field and the
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